Method of dynamically controlling the withdrawal speed during a healing cycle following sticking in a process for the continuous casting of steel

Abstract

On detection of an occurrence of skin sticking in the mould, the withdrawal speed is subjected to a cyclic variation which comprises a ramp from the cruising speed to a reduced, decelerated speed, a healing plateau, and an acceleration ramp from the reduced speed to the cruising speed, measures are taken to determine the ferritic potential (PF) of the steel which is being cast, to determine the gradients (d, a) of one of the two ramps as a function of this ferritic potential, and to determine the length (t r ) of the healing plateau as a function of the difference between the liquidus and solidus temperatures of said steel.

We claim: 
     
       1. Method of dynamically controlling the withdrawal speed in a process for the continuous casting of steel comprising detecting the occurrence of skin sticking in the mould, on detection of an occurrence of skin sticking in the mould, subjecting the withdrawal speed to a cyclic variation which comprises a deceleration ramp from the cruising speed to a reduced, decelerated speed, a healing plateau, and an acceleration ramp from the reduced speed to the cruising speed, characterized in establishing at least the gradient of one of the two ramps as a function of the ferritic potential of the steel which is being cast. 
     
     
       2. Method according to claim 1, further characterized in the step of determining the ferritic potential of the steel which is being cast. 
     
     
       3. Method according to claim 2, characterized in establishing the length (t r ) of the healing plateau is established as a function of the difference between the liquidus and solidus temperatures of the steel which is being cast. 
     
     
       4. Method according to claim 2, characterized (PF) in selecting the value for the ferritic potential of low alloy steel to conform to the formula:   PF=2.5(0.5-%Cp)     where %C p  is the carbon equivalent in the peritectic . reaction, calculated in accordance with the formula:     %C.sub.p =%C+0.02% Mn+0.04% Ni-0.1% Si-0.04% Cr-0.1% Mo     and in that the value selected for the ferritic potential of stainless steel conform to the formula:     PF=5.26(0.74-[%Ni'/%Cr'])     where     %Ni'=%Ni+0.31% Mn+22% C+14.2% N+%Cu       %Cr'=%Cr+1.5% Si+1.4% Mo+3% Ti+2% Nb     
     
     
       5. Method according to claim 2, characterized in that the length of (t r ) of the healing plateau is the ordinate value, to a degree of approximation, of the curves displayed in FIG. 2 at the abscissa point corresponding to the T L  -T S  of the steel which is being cast, and the gradients of the deceleration and acceleration ramps are the ordinate values, to a degree of approximation, of the curves displayed in FIG. 2 at the abscissa point corresponding to the PF of the steel which is being cast. 
     
     
       6. Method according to claim 2, characterized in that the deceleration time(t d ) is of the order of 0.5 to 30 s, the waiting time (t r ) at reduced speed is of the order of 30 to 300 s, and the acceleration time (t a ) is of the order of 60 to 600 s. 
     
     
       7. Method according to claim 2, characterized in controlling the withdrawal speed by means of a computer embodying a program for establishing the ramp speed gradient by a ferritic potential calculation according to the steel which is being cast. 
     
     
       8. (Amended) Method according to claim 2, characterized in that the reduced speed in the healing cycle in meters per minute is substantially equal to the larger of two values: one being 70% of the cruising speed, and the other being the useful length of the mould divided by the length t r  of the healing plateau. 
     
     
       9. Method according to claim 3, characterized in selecting the value for the ferritic potential (PF) of low alloy steel to conform to the formula:   PF=2.5(0.5-%C.sub.p)     where %C p  is the carbon equivalent in the peritectic reaction, calculated in accordance with the formula:     %C.sub.p =%C+0.02% Mn+0.04% Ni-0.1% Si-0.04% Cr-0.1% Mo     and in that the value selected for the ferritic potential of stainless steel conform to the formula:     PF=5.26(0.74-[%Ni'/%Cr'])     where     %Ni=%Ni+0.31% Mn+22% C+14.2% N+%Cu       %Cr'=%Cr+1.5% Si+1.4% Mo+3% Ti+2% Nb.     
     
     
       10. Method according to claim 9, characterized in that the length of (t r ) of the healing plateau is the ordinate value, to a degree of approximation, of the curves displayed in FIG. 2 at the abscissa point corresponding to the T L  -T s  of the steel which is being cast and the gradients of the deceleration and acceleration ramps are the ordinate values, to a degree of approximation, of the curves displayed in FIG. 2 at the abscissa point corresponding to the PF of the steel which is being cast. 
     
     
       11. Method according to claim 10, characterized in that the deceleration time (t d ) is of the order of 0.5 to 30 s, the waiting time (t r ) at reduced speed is of the order of 30 to 300 s, and the acceleration time (t a ) is of the order of 60 to 600 s. 
     
     
       12. Method according to claim 11, characterized in controlling the withdrawal speed by means of a computer embodying a programme for establishing the ramp speed gradient by a ferritic potential calculation according to the steel which is being cast. 
     
     
       13. Method according to claim 12, characterized in that the reduced speed in the healing cycle in meters per minute is substantially equal to the larger of two values: one being 70% of the cruising speed, and the other being the useful length of the mould divided by the length t r  of the healing plateau.